A batter is credited with a double when he hits the ball into play and reaches second base without the help of an intervening error or attempt to put out another baserunner.
Doubles are typically hit either into a gap, down the lines or off the outfield wall. They often score runners from first base -- except for in the instance of a ground-rule double, where the ball bounces into the stands and all baserunners, including the batter, are awarded two bases.
Late in games, teams often employ a "no-doubles defense" when they have the lead, where outfielders play very deep in an effort to prevent extra-base hits.
Watch: Jed Lowrie hits a double.
In A Call
"two-bagger," "two-base hit," "extra bases"